Do you notice the gap in the noun endings? Let us find out what it is!
It is the 5th declension, the smallest and probably the strangest declension of all five.


The masculine words have such endings:


vanduo (water)
vandens
vandeniui
vandenį
vandeniu
vandenyje
vandenie!
vandenys
vandenų
vandenims
vandenis
vandenimis
vandenyse
vandenys!


Note that the en part is considered a part of the stem. Everything else is the endings.
All the words of this declension are masculine, except for two which are feminine:
sesuo (sister)
duktė (daughter)


What is more, the stem is with an er, not with en:
sesuo
sesers
seseriai
seserį
seserimi, seseria
seseryje
seserie!
seserys
seserų
seserims
seseris
seserimis
seseryse
seserys!


Duktė doesn't have an uo:
duktė
dukters
dukteriai
dukterį
dukterimi, seseria
dukteryje
dukterie!
dukterys
dukterų
dukterims
dukteris
dukterimis
dukteryse
dukterys!